1. Field of the Invention
The present invention relates to image pickup apparatuses, image capturing systems, and methods for driving image pickup apparatuses.
2. Description of the Related Art
Recently, image pickup apparatuses, such as digital cameras and video cameras, having a pixel array, such as a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor, including a plurality of photoelectric conversion elements have been widely used. Such image pickup apparatuses generally have an electronic view finder (hereinafter, abbreviated as “EVF”) function for repeatedly displaying images captured using an image pickup element (for recording) to allow users to check images of a subject. This EVF function generally uses a liquid crystal display device included in the image pickup apparatuses. An image quality high enough to satisfy the level required for this function can be obtained using data from some of the pixels of the pixel array in displaying of the images. Accordingly, methods for reading out pixel data while thinning out some (rows or columns) of the pixels of the pixel array are often employed.
Execution of focusing and light metering of a subject at this time using signals fed from pixels (thinned-out pixels) whose data is not output as images has been suggested (see Japanese Patent Laid-Open No. 2005-277513). According to this suggestion, an image pickup apparatus includes a first output unit configured to output pixel signals as voltage values and a second output unit configured to output the pixel signals as current values. The image pickup apparatus uses the output values of the first output unit in displaying of images and uses the output values of the second output unit in focusing and light metering operations. In addition, the first and second output units can operate independently and employ different readout cycles (a driving frequency of the second output unit is set lower than that of the first output unit to improve a signal-to-noise (S/N) ratio).
However, since the first and second output units synchronously operate in a technique disclosed in Japanese Patent Laid-Open No. 2005-277513, the operation performance may not be fully improved.
For example, different asynchronous signals are used as a driving signal of an image pickup apparatus and a driving signal (e.g., a motor driving signal) of an image capturing lens in a focusing operation and a face detection operation. When a focusing operation (e.g., a servo AF operation) or a face detecting operation is carried out in synchronization with the driving signal of the image pickup apparatus, a subject may not be tracked sufficiently.
In addition, when an exposure control operation (e.g., an AE operation) is carried out in synchronization with the driving signal of the image pickup apparatus, a target exposure value of photoelectric converting elements may not be finely adjusted according to the luminance of a subject.